Chemically sensitive field-effect transistors (CHEMFETs) have been developed for the detection of specific compounds in liquid and gaseous environments, such as the ion sensitive CHEMFETs disclosed in U.S. Pat. No. 4,020,830 to Johnson, et al. and U.S. Pat. No. 4,305,802 to Koshiishi.
Other CHEMFETs have been produced that measure the concentrations of components in a gaseous state, as for example the devices disclosed in U.S. Pat. No. 3,719,564 to Lilly, Jr., et al. and described by Shimada, et al. in U.S. Pat. Nos. 4,218,298 and 4,354,308, and the suspended gate field-effect transistors (SGFETs) described by Jiri Janata in U.S. Pat. Nos. 4,411,741 and 4,514,263. However, such devices do not provide linear response to the desired components resulting in lower measurement accuracy. These devices are also relatively complex and costly to manufacture due to their multiple junctions and diffusion regions.
CHEMFETs in general, and SGFETs in particular are also not well suited to the detection of combinations of specific compounds or of specific compounds in the presence of other potentially interfering chemical species. Combinations of discrete SGFETs with sensitivities to different compounds have been proposed in efforts to address such deficiencies. For example, in U.S. Pat. No. 4,368,480 to Senturia multiplexed CHEMFETs provide logic elements with varying on-off duty cycles. Nevertheless, such combinations are even more difficult and costly to manufacture than individual sensors.
CHEMFETs, such as those exemplified by U.S. Pat. No. 4,411,741 to Janata and U.S. Pat. No. 4,486,292 to Blackburn, are two-port (i.e., three terminal) devices that monitor environmental fluid concentration by measuring current flow within the semiconductor between the source and drain, i.e., drain current. The background of the Blackburn patent states in-part that the term CHEMFET embraces diode-type devices which feature conductivity modulation similar to that of CHEMFETs.
However, diodes are one-port (i.e., two terminal) devices and therefor have no drain or drain current to measure. Thus, if a MIS diode is to be used to monitor fluid concentration, another device parameter must be employed.
Those ordinarily skilled in the sensor art would measure an MIS diode's capacitance to monitor environmental fluid concentration since capacitance is proportional to surface potential which is in turn proportional to fluid concentration. But the known surface physics of an MIS diode makes clear that such a measurement would not be successful.
The relevant surface physics of MIS diodes may be best appreciated from the renowned text entitled Physics of Semiconductor Devices, (Second Edition--1981) by S. M. Sze. Chapter 7 of that text presents in Sections 7.1 and 7.2 (at pages 362-379, a copy of which accompanies this Amendment) a thorough discussion of MIS diode construction and operation.
The response of MIS diode capacitance to changes in surface potential can be understood by first referring to FIG. 5 (on page 369) which illustrates the variation of space-charge density (Q.sub.s) in the semiconductor as a function of the surface potential (.psi..sub.s) for an exemplary p-type silicon. As seen in that Figure, changes in space-charge density may be broadly classified as occurring in three modes: accumulation, depletion and weak inversion, and strong inversion.
Only in the depletion and weak inversion mode do changes in surface potential (which are proportional to changes in fluid concentration) produce changes in depletion region depth, which in turn alters space-charge capacitance. However, in this mode the magnitude of changes in depletion region depth vary as the square root of changes in surface potential. Moreover, the skilled artisan will appreciate that capacitance varies in a complex manner with temperature. In other words, operating a MIS diode in this mode would result in a poor, temperature variant fluid concentration sensor.
In the strong inversion mode the skilled artisan would recognize that capacitance cannot be used to monitor fluid concentration because, as depicted in FIG. 9 on page 374, for surface potentials this great the depletion region reaches a fixed minimum depth (W.sub.m). Since capacitance is proportional to the depth of the depletion region within the diode semiconductor which under these conditions does not change, diode capacitance could not reflect fluid concentration changes.